Under the promotion of Moore's Law, the semiconductor industry has developed rapidly during the past decade. With the development of more complicated process technology, a well controlled process thermal budget is becoming more important for the semiconductor fabrication. For example, when the integrated circuit enters the deep sub-micrometer and nanometer scale, the critical dimension reduces continuously. However, impurity ions in the doped region formed by ion implantation or diffusion accumulate energy during the repeated thermal treatments. This energy may be enough for the impurity ions to free from the substance of the substrate and diffuse into the neighboring regions of reduced spacing, and this always results in device failure.
During the semiconductor manufacturing procedure, most of the heat comes from the heating steps like thermal oxidation, thermal annealing, thermal diffusion, vacuum evaporation, and CVD, etc, since all heating steps generally involve heating treatment at a high temperature for a long time. However, as for the heating procedure, sometimes it is required to adjust the primary process duration to ensure a thin film with a certain thickness on the target object. For example, as for the thermal oxidation, in case that dry oxygen oxidation is performed at 800° C. on (100) crystallographic plane of Si, when the moisture content in the oxidant atmosphere is less than 1 ppm, an oxide layer with a thickness of 300 Å is formed by oxidation for 700 min A longer time is required to form a thicker oxide layer. The thickness of the oxide layer shows substantially a linear relation with the required time, which to some extent also implies that a thicker oxide layer would require more thermal budget.
It is important that the primary process is mostly a treatment step with the highest temperature. For instance, as for the fabrication of silicon oxide, the primary process is, for example, oxidation, in which the temperature ranges from 800 to 1300° C. or so, the duration of heating treatment is generally in the order of ten minutes, several hours, or a dozen of hours. On the contrary, the duration of thermal annealing is generally in the order of some minutes or some seconds, so that annealing at 300° C. can essentially remove defects which are introduced by the low-dose damage of Sb+ implanted into Si. The duration of CVD is generally the order of some minutes, with the temperature ranging from 300 to 750° C. or so. Furthermore, among the step procedures for heating in the furnace, the primary process takes a relatively long time. As can be seen, the thermal oxidation can best represent the primary process which is performed at high temperature for a long time, and to a great extent determines the thermal budget of the semiconductor process. Therefore, changing the treatment duration of the primary process will eventually result in significant variation in the thermal budget of the device.
Furthermore, when the thermal budget shows a relatively large variation, further process on the wafer may cause the wafer subject to a total thermal energy beyond the requirements to maintain the stability of the device. That is, when the thermal budget increases suddenly, if the subsequent process can not leave adequate margin according to the thermal budget, the total thermal budget of the device may be exceeded and the diffusion is out of control to result in the device failure.
In a word, there is a need for a method for optimizing heating procedures which is capable of maintaining a stable thermal budget, in order to reduce manufacturing cost and save the process duration.